In recent years and continuing to the present, attention is now focusing on security when a user accesses confidential information to use information terminals such as computers and mobile phones or enters buildings. As means for authenticating whether the user is authorized to access information, the user uses a magnetic stripe and an IC chip embedded in a card, a password that the user memorizes, biometric authentication using the physical characteristics of the user, or the like. However, the user may lose a card and forget a password. Moreover, the password may be leaked to others. On the other hand, when the user uses biometric authentication, he/she is free from the risks of losing the card or forgetting the password. In addition, it is hard for the user to imitate the information of others with the biometric authentication.
Nevertheless, in the personal authentication device using the vein pattern of a finger or palm as biometric information, the image of the vein pattern is taken with the finger or palm set in position by a guide unit (see, e.g., Patent Document 2). The living body is thus set in a predetermined position, because accuracy in authentication processing is degraded or errors are caused if the taken image of the vein pattern is not almost the same in size as that of a registration pattern. In this case, however, the living body must contact the guide unit and a device becomes large as a whole with the installation of the guide unit. Therefore, when the personal authentication device is used by an indefinite number of users, a device that the users are not required to touch is desired from the viewpoint of hygiene. Recently, information terminals such as mobile phones and notebook computers have been downsized and made thinner. Therefore, if the personal authentication device is large, it is hard to be installed in the information terminals.
Furthermore, Patent Document 1 discloses a personal authentication device capable of performing authentication without causing the finger of a test subject to be set in a predetermined position. This personal authentication device includes a registration unit and an authentication unit. The registration unit acquires three-dimensional biometric information including ones related to veins, skin contour, and joint positions of the finger of a registerant, and sends it to the authentication unit. The authentication unit measures the positions of parts of the finger of a person to be authenticated and shows them as coordinates and corrects the three-dimensional biometric information received from the registration unit so as to correspond to the states of the position, inclination, and joint curvature, etc., of the finger of the person to be authenticated. In this manner, the personal authentication using the three-dimensional biometric information after being corrected and the biometric information received by the authentication unit is performed. However, the authentication unit is required to have a position measuring unit that transmits light or ultrasonic waves to the finger and measures its reflection time, a unit that performs complicated correction processing for the three-dimensional biometric information, or the like. In addition, the registration unit is required to have a unit that uses X-ray measurement, magnetic resonance measurement, light measurement, ultrasonic wave measurement, or the like, to acquire the three-dimensional biometric information. Therefore, it is hard to reduce the size and costs of the device.    Patent Document 1: JP-A-2007-219    Patent Document 2: JP-A-2006-107401
When the image of a vein pattern of a finger is taken without causing the finger to be set in a predetermined position by the guide unit or the like in the personal authentication device that takes the image of the vein pattern of the finger so as to be used for authentication processing, the most important problem is that a distance between a test subject, the device and the position of the finger are likely to fluctuate at the time of taking the image.